jdanyow/index.html

## index.html
<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">

  <!-- this breaks iframe transparency: -->
  <meta name="color-scheme" content="light dark">
  <!-- remove the line above and the iframe background becomes transparent as expected -->

  <title>Demo</title>
</head>
<body style="background-color: green">
  <h1>Hello world!</h1>
  <iframe></iframe>
  <script>
    const fdoc = document.querySelector('iframe').contentWindow.document;
    fdoc.head.innerHTML = `
      <style>
        html,body { background-color: transparent; }
      </style>
    `;
    fdoc.body.innerHTML = `
      <h1>Hello world!</h1>
    `;
  </script>
</body>
</html>
	<!doctype html>
	<html lang="en">
	<head>
	<meta charset="utf-8">

	<!-- this breaks iframe transparency: -->
	<meta name="color-scheme" content="light dark">
	<!-- remove the line above and the iframe background becomes transparent as expected -->

	<title>Demo</title>
	</head>
	<body style="background-color: green">
	<h1>Hello world!</h1>
	<iframe></iframe>
	<script>
	const fdoc = document.querySelector('iframe').contentWindow.document;
	fdoc.head.innerHTML = `
	<style>
	html,body { background-color: transparent; }
	</style>
	`;
	fdoc.body.innerHTML = `
	<h1>Hello world!</h1>
	`;
	</script>
	</body>
	</html>